1. Field of the Invention
The present invention relates to fiber optic networks and multi-channel communication systems.
2. Related Art
Modern communication systems increasingly rely upon fiber optic networks to carry increasing amounts of data between sites. The use of multiple optical carriers, also called channels, over the same optical fiber increases capacity. Wavelength division multiplexing (WDM) allows multiple channels to be carried on a fiber in different carrier wavelengths. Attenuation and dispersion in an optical fiber limit the distance an optical signal can travel without amplification and/or dispersion compensation.
In commercial optical fibers, there are two infrared wavelength windows or bands at which the fiber material offers minimal attenuation. One window is generally called the "1310 nm window" and includes a wavelength band between approximately 1150-1385 nm (nanometer) with a minimum loss of about 0.4 dB/km. The other window includes longer wavelengths in a range between approximately 1500-1600 nm and has minimum attenuation of about 0.2 dB/km (decibel/kilometer). The window between about 1520 to 1560 nm is often amplified by erbium-doped materials and thus has been called the "erbium band" or "erbium window."
Because of the lower loss and corresponding reduction in line amplification required, the telecommunication industry has focused upon devices and fibers to support operation at around 1550 nm, especially in multi-channel, WDM applications. The 1310 nm band was essentially abandoned as new fibers, semiconductor lasers and receivers were developed to support 1550 nm WDM operation. Heretofore, commercial systems have primarily employed the 1310 nm window for single-channel communication.
In order to increase the utilization of an optical communications fiber, wavelength division multiplexing (WDM) is employed to send multiple optical carriers along the fiber, each at a different wavelength. Engineers are striving to maximize the capacity of the erbium band in a communications network by putting as many wavelengths as possible onto a fiber. While two-wavelength and four-wavelength systems are fairly common, the telecommunications industry is planning for ways to crowd eight or sixteen channels at 100 GHz or 50 GHz spacing within the narrow erbium band. This presents significant challenges in transmitter stability, receiver selectivity, ease of line amplification and equalization, and avoidance of non-linear interference effects such as four-wave mixing (FWM). Thus, only a small number of WDM channels can be effectively supported in the erbium band of an optical fiber network without sacrificing reliable, high-quality communication. For example, according to one International Telecommunication Union (ITU) standard, a 100 Gigahertz (GHz) spacing is provided between channels to maintain signal separation and quality. This 100 GHz spacing translates to a wavelength range of approximately 0.8 nm, meaning only 40 WDM channels fit within an erbium fiber band. However, if each optical carrier is modulated at high data bit rates, such as 10 Giga-bits/second (Gb/s), a 200 GHz spacing is preferably used between channels to avoid crosstalk. As a result, only sixteen channels with 200 GHz spacing can be used effectively in an operating window within an erbium band of approximately 1530 to 1561 nm.
Group velocity dispersion also complicates WDM deployment in an erbium band because a given fiber exhibits a sloped dispersion characteristic as a function of wavelength. Thus, a fiber can exhibit small dispersion values only over a subset of the wavelengths available in the erbium band. For carrier wavelengths distant from the zero-dispersion wavelength (.lambda..sub.0), the dispersion effect must be compensated at intervals along the fiber to assure reliable signal reception. This further limits the number of channels which can be used in the erbium band for reliable, high-quality communication
One of the earliest types of single-mode fiber that came into widespread use was retrospectively dubbed Non-Dispersion Shifted Fiber (NDSF). For example, such fiber has a zero-dispersion wavelength .lambda..sub.0 around 1312 nm and a zero dispersion slope S.sub.0 of about 0.090 ps/nm.sup.2 -km. See, e.g, CORNING.RTM. SMF-28.TM. CPC6 single-mode optical fiber, Product Information, 1997, pages 1 and 3. Further, the NDSF fiber can have a positive average dispersion across the erbium band. In practice, designers have been able to compensate for this by installing negative-slope fiber at intervals along an optical link.
A more recent fiber, the Dispersion-Shifted Fiber (DSF), was formulated such that the .lambda..sub.0 falls at 1550 nm, making it ideal for transmitting at that wavelength. However, one drawback is that if several WDM carriers are crowded around this wavelength and then launched with sufficient optical power density into a common fiber, then the carriers will interact through Four-Wave Mixing (FWM) due to non-linearity of the fiber material.
The capacity of fibers and fiber networks needs to be increased. Multiple channels need to be added without sacrificing the reliability and quality of voice and data communication. The overall bandwidth of a single-mode fiber, such as, an NDSF fiber and/or a DSF fiber, needs to be optimized. A dense WDM window is needed in which many channels can be used to support multi-channel communication over single-mode fiber.